Barium titanate (BaTiO.sub.3) ceramics having a perovskite crystal structure have been widely used as capacitor materials and the like, since they have high resistivities and superior dielectric properties. Strontium titanate (SrTiO.sub.3) becomes cubical at about a temperature higher than 110K, and is paraelectric. Ceramics mainly made from strontium titanate have lower dielectric constants than do the barium titanate ceramics. However, the strontium titanate ceramics have better temperature characteristics and less dielectric loss than do the barium titanate ceramics.
The curie points of ceramics can be shifted by employing an additive such as barium, lead or the like, thus providing paraelectric ceramics with a high dielectric constant at room temperature. These ceramics have been widely used for high frequency range and high voltage capacitors. Because complex perovskite compounds such as Pb(Mg.sub.1/3, Nb.sub.2/3)O.sub.3 and Pb{(Mg.sub.1/3, Nb.sub.2/3).sub.1 -Y Ti.sub.Y }O.sub.3 have higher dielectric constants and better direct current biases than do barium titanate dielectrics, they are used for small multilayer capacitors with high capacitance and the like.
As the demand for small, lightweight electronic components and highly integrated semiconductor devices increases, research in forming thin films of oxide materials having a perovskite structure and large dielectric constants such as barium titanate dielectric materials, strontium titanate dielectric materials, Pb{(Mg.sub.1/3, Nb.sub.2/3).sub.1-Y Ti.sub.Y }O.sub.3 dielectric materials and the like has been actively carried out in order to manufacture small high-volume capacitors.
As a method of forming thin layers, a sputtering method has been conventionally applied. (For example, the method is disclosed in K. Iijima et al., J. Appl. Phys., vol. 60, No. 1, pp.361-367 (1986).) However, this method is slow in forming layers (&lt;10 nm/min), requires expensive monocrystal substrates such as MgO and is poor in controlling the compositions of layers.
A sol-gel method, which has been intensively researched recently, on the other hand, is excellent in controlling the compositions of layers and suitable for forming multicomponent thin layers. However, this method provides layers with poor quality and coverage over uneven surfaces so that it cannot be applied as an industrial manufacturing technique.
A CVD (chemical vapor deposition) method is excellent in controlling the compositions of layers, and can form layers on substrates having large surface areas. The method also has a good property of covering uneven surfaces. Therefore, this method can be applied as a method of forming perovskite dielectric thin layers. (See M. Okada et al., Jpn. J. Appl. Phys., vol. 28, No. 6, pp. 1030-1034 (1989).) Compared with the sputtering method, the CVD method can form layers at high speed. However, the speed of forming a dielectric thin layer by the CVD method is still about 3 .mu.m/h. For instance, in order to form a 2 .mu.m thick dielectric layer (for example, BaTiO.sub.3), the method takes about 40 minutes. (See B.S. Kwak et al., J. Appl. Phys., vol. 69, No. 2, pp. 767-772 (1991).)
Layers should be formed at a higher speed so that a thin film capacitor can be manufactured at lower cost. When a dielectric thin layer is directly formed on a metal electrode substrate or a metal thin layer by a sputtering method or a CVD method, a layer with poor crystallinities (initial growth layer) is formed in the early stages of film growth. Due to the formation of the initial growth layer, the electric characteristics of the thin film capacitor are lowered as the capacity of the capacitor is increased by thinning the dielectric thin layer.
On the other hand, a plasma-enhanced CVD method making use of plasma activity and CVD reaction can form layers at low temperature and high speed by dissolving and enhancing source materials with an active plasma. (For instance, see E. Fujii et al., Jpn. J. Appl. Phys., vol. 32, No. 10B, pp. 1527-1529 (1993) and A. Tomozawa et al., Journal of Magnetic Society of Japan, vol. 17, No. 2, pp. 319-322 (1993).) However, like the sputtering method and the CVD method, the plasma-enhanced CVD method forms a layer with poor crystallinity (initial growth layer) in the early stages of film growth when a dielectric layer is directly formed on a metal electrode substrate or a metal thin layer.